Arrangement for reducing equalizing oscillations



Mai-ch 2, 1943. H. MEYER 2,312,571

ARRANGEMENT FOR REDUCING EQUALIZING OSGILLATIONS Filed July 16, i941Patented Mar. 2, 1943 ARRANGEMENT FOR REDUCING EQUALIZ- ING OSCILLATIONSHans Meyer, Baden, Switzerland, assignor to Aktiengesellschaft Brown,Boveri & Cie., Baden,

Switzerland,

Application July 16, 1941, Serial No. 402,713 In Switzerland December24, 1940 11 Claims.-

In transformers where the star point is not grounded, either directly orindirectly it is known that equalizing oscillations are excited whenmulti-pole surges occur, Depending on the shape and duration of thesurge the surge voltage at the star point can as a result of thisoscillation process rise to twice its value at the ingoing terminals ofthe transformer. As a result of this phenomena the insulation of theWinding, particularly in the vicinity of the star point, is stressed toan undesirably high degree and any apparatus such as currenttransformers, are suppression coils and the like, which may be connectedto the star point, must be provided with a stronger insulation toprotect them against these equalizing voltages.

The prior attempts to limit the voltage rise at the ungrounded starpointof a multiphase transformer have been based upon the idea of bleedingoff the high frequency oscillating voltage to ground through acondenser, a seriesresonant circuit or a resistor. These expedients arenot entirely satisfactory as the oscillation-limiting impedances were inshunt with are suppression coils, current transformers and otherequipment that is normally connected between the transformer starpointand ground. In general, it was not possible to design theoscillationlimiting impedance for optimum performance as such designwould adversely affect the operating characteristics of the otherequipment.

The present invention concerns an arrangement for reducing equalizingoscillations which occur with multi-pole surges, particularly star pointoscillations in transformers, without the disadvantages which accompanythe arrangements used hitherto. The invention is characterised by thefeature that a composite impedanceis connected into a circuit of the ofan embodiment of the invention as applied to a transformer having anungrounded starconnected primary-winding; Fig. 2 is a similar circuitdiagram of another embodiment in which the star point of the transformerprimary is grounded; and Fig. 3 is a circuit diagram of an embodiment inwhich the surge reducing impedances are located in a measuring circuit.

Fig. 1 shows a transformer with the primary winding star connected.Multi-pole surges occur at the terminals u, '0, w of the primarywindings and result in high frequency oscillatory Voltages at theneutral point that is not grounded but coupled to ground through thedistributed capacity to ground, represented by the dotted line condenserC, of the transformer windings. The transformer secondary may be forinstance a polygon-connected winding, and in the embodiment illustratedis a delta-connected winding into which the composite impedance Z,comprising an inductance or choke coil and resistance in parallel, isconnected. This impedance is then according to the invention practicallya short-circuit for currents of operating frequency. When multi-polesurges reach the primary winding, the induced high-frequency volt- 7ages of the several phases of polygon arrangement add together and forcethe equalizing current over the impedance Z. This arrangement thusprovides a very elfective damping of the equalizing oscillations due tothe fact that the impedance is composed of partial impedances in such amanner that it has substantially the charactor of an ohmic resistance ofsuitable high magnitude for the equalizing oscillation frequency. Thisarrangement has the advantage, as compared with one where acorresponding number of damping impedances are associated with thephases, that the composite impedance Z has a negligible ohmic resistanceat the frequency of the operating current, whereby the normal flow ofpower current established substantially no voltage drop across theterminals of the impedance. The impedance Z can of course also bedivided up into a number of elements corresponding to the number ofphases and connected alternately between adjacent. phase windings inorder to obtain a symmetrical assembly.

Equalizing oscillations can be reduced very effectively or evencompletely suppressed when the equalizing voltages occurring at theimpedance are utilized in the circuit between the star point and ground.According to the arrangement illustrated in Fig. 2 this is achieved byconnecting both terminal points of the polygon, which is closed by theimpedance Z directly or by means of additional impedances R, with thesecondary winding of a transformer T located between the star point andground in such a manner that when a voltage drop occurs at the impedanceZ the voltage induced in the primary winding of the star-pointtransformer opposes the equalizing voltages due to the differentialcoupling. The impedance Z may be connected directly to the star-pointtransformer only if the secondary winding of the transformer can beshort circuited in service. When the indirect connection is employed itis expedient to connect the damping part of the impedance Z in serieswith the secondary winding of the starpoint transformer, The impedance Zshown in Fig. 2 is then for instance a choke coil shunted by a condenserand resistance in series, and the secondary of the transformer T isconnected across the composite impedance Z through the dampingresistance R.

In order to increase the favourable effect of the differential couplingit is an advantage to select the ratio of the number of phase coils ofthe primary and polygon winding so as to be larger than that of thenumber of coils of the primary and secondary windings of the star-pointtransformer T used for the purpose of compensation.

The frequency dependent impedance Z can, however, also be employeddirectly to effect a reduction of the equalizing oscillations in thecircuit between the star point and ground of the transformerarrangement. The impedance can be switched into this circuit or as shownin Fig. 3 connected to the terminals of the secondary winding of astar-point transformer and should be so dimensioned that its value is ashigh as possible for the operating frequency, but at the same timeassumes the most favourable value as a damping resistance for theequalizing frequencies. The star-point transformer T can with advantageconsist of an existing arc suppression coil or a voltage transformerused for measuring the voltage. In the case of the latter impedances a:or similar elements should be located in the measuring circuit as shownin Fig. 3 in order to render the circuit impassable for equalizingcurrents.

In order to achieve the best results the frequency dependent impedance Zshould consist of resonance circuits and damping elements. Tests haveshown, however, that particularly when the arrangement according to Fig.1 is employed it is generally sufficientif the impedances are formed bya choke coil and a resistance connected in parallel. The choke coilshould then be so dimensioned that it offers a negligible resistance tothe operating currents, but at the same time such a large reactance forthe higher frequency equalizing currents that these are forced to flowover the damping resistance connected in parallel. This difference ineffect, which forms the basis of the present invention, can be stillfurther improved, without any appreciable additional cost, if acondenser of suitable size is connected in series with the dampingresistance.

The damping of the equalizing process determined mainly by the size ofthe ohmic resistance employed, should be so selected that the course ofthe equalizing currents for all loads between no-load and full load,that is at equalizing frequencies which differ slightly from each other,approaches the most favourable case of aperiodicity. The equalizingprocess is then maintained within controlled limits independent of theshape and duration of the surges which occur. By this means it ispossible to dispense with a strong insulation for the primary winding,particularly in the vicinity of the star point or of any apparatus whichmay be connected to this point.

I claim:

1. A multiphase power transformer comprising a star connected primarywinding having an ungrounded star point, secondary. windings, and meansreducing the magnitude of high frequency oscillations arising frommultiphase surges; said means comprising a composite frequency-variantimpedance connected in circuit with one of said windings in the paths ofpower current and of the oscillating current, said impedance being ofnegligible magnitude at the frequency of the power current and of highmagnitude at oscillation frequencies.

2. A multiphase power transformer as recited in claim 1, wherein saidsecondary windings are polygon connected and closed through saidcomposite impedance.

3. A multiphase power transformer as recited in claim 1, wherein thesecondary windings are three-phase and delta connected in series withsaid composite impedance. 1 4. A multiphase power transformer as claimedin claim 1, in combination with a circuit connected between said starpoint'and ground, and means coupling said circuit to said compositeimpedance to establish across the same an oscillatory voltageopposingthe equalizing oscillatory current flow through said compositeimpedance.

5. A multiphase power transformer comprising a star connected primarywinding, a polygon connected secondary winding closed through afrequency-variant composite impedance, said impedance being ofnegligible magnitude at the power current frequency and of highmagnitude at the frequency of equalizing oscillations arising frommultiphase surges, and a transformer hav-" ing a primary windingconnected between the star point of said first primary winding andground, the secondary winding of said second transformer being connectedto said impedance to establish across the same an oscillatory volt-'-age opposing the equalizing oscillatory current flow through saidimpedance.

6. A multiphase power transformer as recited in claim 5, whereinsaidcomposite impedance comprises a choke coil in parallel with aresistin claim 5, wherein said second primary winding is an arcsuppression coil.

10. A multiphase power transformer as recited in claim 5, wherein saidsecond transformeris avoltage transformer.

11. A multiphase power transformer as recited in claim 5, wherein saidsecond transformer is a voltage transformer, and the measuring circuitconnected across the secondar winding thereof includes impedancessuppressing the flow of oscillatory current in said measuring circuit.

HANS MEYER.

